A new sterol, stigmast-5-ene-3beta,17alpha-diol (1), together with six known compounds, stigmast-5-ene-3beta-yl formate (2), stigmast-5-ene-3beta,7alpha-diol (3), stigmast-5-ene-7alpha-methoxy-3beta-ol (4), stigmast-5-ene-3-one (5), 3beta-sitostanol (6), and 3beta-sitosterol (7), was isolated from the fruits of Syzygium siamense, of which compound 2 is reported for the first time from a natural source. Their structures were elucidated by spectroscopic methods. The isolated compounds (1-7) were evaluated for their cytotoxic activities against human oral epidermoid carcinoma cancer (KB), human breast cancer (BC), and human small cell lung cancer (NCI-H187) cell lines.